1. Field of the Invention
The present invention pertains to the processing of seismic data, such as seismic imaging reflection data. More particularly, the present invention is related to techniques for preparing such seismic data for migration, and is applicable for use with two-dimensional (2-D) and three-dimensional (3-D) data.
2. Brief Description of Prior Art
Seismic exploration is carried out by generating acoustic waves by one or more sources which direct the wave fronts into the earth's subsurface. Wave fields reflected by subsurface structures, or horizons, are received at the surface by detectors, or receivers, such as geophones. The sources may be explosive devices, vibrating devices, falling weights, air guns or the like. Electrical cables connect the receivers to a monitor which records the electrical signals produced by the receivers in response to the detected acoustic waves. For seismic exploration conducted through a body of water, the receivers are hydrophones positioned along a streamer.
Signals from an array of receivers comprising a single line may be utilized to provide information about subsurface structures generally lying along a vertical plane. With such information, a 2-D vertical, seismic section of the subsurface may be produced, in pictorial form, for example. A receiver and source array comprising a multiplicity of generally parallel, and relatively closely-spaced lines of receivers and sources provides data which may be utilized to produce a 3-D representation of subsurface structures. A 3-D representation may be provided in cube form, for example, or any number of 2-D seismic sections may be generated from the 3-D data. Also, a horizontal slice, for example, corresponding to a chosen depth below the earth surface, may be generated from the 3-D data.
The seismic sections are constructed based on output signals from the receivers in response to detected acoustic waves. The output signal from a single receiver is presented as a trace in the form of a wave signal time plot. A single wave-producing activation of a source, called a shot, results in a collection of traces equal to the number of receivers. Aligning the traces in the order of the positions of the receivers in the line, for example, can produce a rudimentary seismic section. Such a section is thus a display of wave form data utilizing time along a vertical axis, and distance (locations of the sources) along a horizontal axis. More meaningful seismic sections are produced by processing the data represented by the traces, and combining data from multiple shots and/or from shots using various combinations of sources and receivers, for example. Various techniques are used to combine and process seismic data to make seismic sections and cubes more reliable and accurate sources of information concerning subsurface structures, thereby enhancing the usefulness of such data presentations in the quest for oil and gas deposits, for example.
One of the techniques utilized in processing seismic data is to combine traces produced from two or more shots wherein the midpoint between the source and the receiver in each case is the same, although the offset, or source-to-receiver distance (SGD), may be different in each case. This technique, called common midpoint (CMP) stacking, adds the data from multiple reflections from the same, or nearly the same, subsurface point via different paths, while noise that occurs at different times on the multiple traces is not added. The selection of a limited number of traces from all traces obtained is a gather; the selection of all traces of a CMP, such as to be combined in a stack, is a CMP gather.
Since the wave fronts received by receivers with increased SGD must travel greater distances under the surface than is the case for smaller SGD, the detection of a reflected wave from a given subsurface point occurs at a later time for the same shot in a trace obtained from a receiver with a greater SGD than is the case for a trace obtained from a receiver with a smaller SGD. The result is that, in a seismic section constructed from such traces, the wave pattern among the various traces corresponding to the same subsurface point appears at later points on the time scale. This phenomenon, called normal moveout (NMO), must be considered in processing the data for construction of useful sections. Appropriate adjustment, or compensation, for NMO is usually made.
Another phenomenon that must be considered in processing data for production of seismic sections useful for analysis is the effect of dip in the subsurface structures. If the subsurface reflecting structure is flat and horizontal, the reflecting point is directly under the midpoint of the SGD. This is true for all traces in a CMP gather for that point. However, if the reflecting surface is curved or tilted, or exhibits a dip, the reflecting point for a single trace will generally be shifted along the reflecting surface, and will not appear directly under the SGD midpoint. Further, for different values of SGD in the same CMP gather of traces, the amount of shifting of the reflecting point from under the midpoint will be different, depending upon the value of the particular SGD in question and the amount and direction of dip exhibited by the reflecting structure. Also, since the variation in wave paths due to dip is accompanied by a variation in the actual depth of reflecting points the velocity of the traveling wave fronts may vary with variations in depth. A wave reflected at one reflection point may have exhibited a different set of velocity values than a wave reflected at a point at a different depth. Differences in wave velocities exhibited by different waves, whose traces are collected in the same CMP gather, may also be due to the different waves reflected back to the surface having passed through different subsurface formations to and/or from the different points of reflection. This effect of shifting of reflection points due to dip in the reflecting subsurface structure is called reflection point smear, and can have significant effects on velocity analyses based on seismic sections if the smear is sufficiently large. Consequently, reflection point smear must normally be addressed in processing seismic data as well, utilizing dip movement (DMO) correction.
To produce a seismic section, as effectively a plot of depth, rather than time of signal reception, versus horizontal location, for example, data from the reflecting horizons must be assigned proper locations in the section. This process is generally accomplished on the basis of known average velocities of reflected waves, and is referred to as migration. Normally, without migration, the trace wave structures are positioned in a section such that the section fairly represents the real subsurface when horizons are flat and all the dip is relatively small. Unmigrated data in a section tends to deviate more from the actual representation of the real subsurface as the magnitude of dip increases. If the data have been migrated, each trace in a seismic section in 2-D, or in a cube in 3-D, may be considered to represent subsurface conditions directly below that trace's assigned position on the earth's surface. Migration thus improves the reliability of interpreting seismic data.
There are several methods commonly used for migrating seismic data after CMP stacking has been performed. There are several known methods for migrating seismic data before stacking, that concurrently perform the function of stacking, taking unstacked data as input and producing zero offset migrated data as output. The functions of stacking and migration are accomplished inseparably, at the same time. Such methods of joint stacking and migration are very sensitive to uncertainties in the velocity information. Obtaining an acceptable result using such processes may entail several lengthy iterations.
It is desirable and preferable to migrate seismic data without stacking, so that velocity analysis and other procedures can be performed on the migrated traces. This is particularly true in the case of 3-D seismic data. Such a method should preserve the identity of unstacked seismic data so that velocity determination can take place after migration but before stacking. There are not many known methods for migrating unstacked seismic data without simultaneously reducing the data to stacked seismic traces. One known method is to organize the data into common offset subsets and to apply to each subset NMO correction followed by DMO correction. Such common offset data can then be migrated as if they were zero offset, and the results sorted back into CMP gathers.
A seismic trace may possess, or be assigned, attributes in addition to time and position. For example, every recorded trace has a particular value of offset SGD. It is common practice to assemble groups of seismic traces having the same value of some attribute in order to exploit some benefit of processing these traces concurrently. For example, common offset 2-D sections or 3-D cubes are routinely employed in seismic processing.
The migration of common offset data is inconsistent with the wave equation. The wave equation yields methods for the downward continuation and migration of unstacked seismic data in common source/common receiver order, or in CMP order. Applying any of these methods to a collection of data requires that migration move information from one offset to another. If downward continuation proceeds to the depth of a particular reflector, all of the recorded information associated with that reflector moves to zero offset. The wave equation provides no method for the downward continuation or migration of data associated with a fixed offset, except for zero offset. Migration methods for the ease of zero offset depend upon the special assumption that downgoing and upcoming ray paths are identical. In general, therefore, common offset migration must rely upon a process to make the common offset section or cube tantamount to zero offset prior to migration. Compared to the wave equation methods that are now standard for full prestacked migration and for zero offset migration, common offset migration must employ additional approximations prior to migration. A known method to accomplish this preprocessing is to employ DMO correction. DMO processing is conceptually the equivalent of prestack migration, wherein all data are contracted to zero offset, followed by zero offset modeling, that is, migration run backward to produce the kinematic equivalent of unmigrated zero offset data. The overall net process can be applied in a single step to common offset data, and the output then behaves like zero offset data as far as migration and downward continuation are concerned. The validity of the migration is dependent on the validity of the DMO processing.
It is desirable and advantageous to provide a technique for prestack migration without including the expensive DMO processing step. It is an object of the present invention to provide an economical alternative to common offset migration that is independent of DMO processing.